1. Technical Field
The present disclosure relates generally to the logging of subsurface formations surrounding a wellbore using a downhole logging tool, and particularly to determining the maximum depth of investigation of measurements made in the formations.
2. Background Art
Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic tools, nuclear tools, and nuclear magnetic resonance (NMR) tools, though various other tool types are also used.
Early logging tools were run into a wellbore on a wireline cable, after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, the need for information while drilling the borehole gave rise to measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, and NMR distributions. MWD and LWD tools often have components common to wireline tools (e.g., transmitting and receiving antennas), but MWD and LWD tools must be constructed to not only endure but to operate in the harsh environment of drilling.
Prior art tools and methods have focused on determining and displaying (mapping) the distance between a measurement sensor carried on the tool and a bed boundary. Identification of a bed boundary is typically characterized by a change in one or more petrophysical properties of the formation. Various techniques and workflows exist to evaluate the distance to the boundary, but none of those allow one to determine the formation volume investigated by deep and azimuthal measurements in the absence of a recognizable bed boundary, or in the case of a measurement that reads beyond a relatively near bed boundary deep into an adjacent formation layer.